In power source devices in for example electric vehicles, an arrangement is adopted whereby a large number of secondary batteries are connected in series to obtain a prescribed drive voltage. In order to ensure reliability and stability of such power source devices, it is necessary to constantly monitor the respective conditions of each of the secondary batteries. For this purpose, it is known to provide a battery voltage measurement device for measuring the voltage at both terminals of each secondary battery.
In such a prior art battery voltage measurement device of this type, as shown in FIG. 4, to each secondary battery 11 there are connected a differential amplifier 12 for obtaining output voltage corresponding to its two terminal voltages and a gain trimmming amplifier 13 for performing gain trimmming of this output voltage. Further, as shown in FIG. 5, the output voltage from each gain trimming amplifier 13 is converted into a digital value by an A/D converter 14 respectively provided corresponding thereto and the digital signal corresponding to the voltage of the two terminals of each secondary battery 11 is input to a microcomputer 15, and the voltage of each secondary battery 11 is thereby detected. As shown in FIG. 4, all the differential amplifiers 12 have their power source terminals and ground terminals respectively connected in common, and power source voltage Vcc is applied by connecting a common control power source between their power source terminals and ground terminals, and their ground terminals are connected to a common ground GND.
The reason for performing respective gain trimmming by connecting gain trimmming amplifiers 13 to each differential amplifier 12 is as follows. In order to achieve digital conversion of the output voltages of the differential amplifiers 12 so that these can be sent to microcomputer 15, the gains of differential amplifiers 12 must be matched with the voltage range of the A/D converter 14 in which input is possible. For example, if the detection range of the battery voltage is 0 to 20V and the input voltage range of A/D converter 14 is 0 to 5V, the battery voltage at differential amplifier 12 must be output multiplied by a factor of 1/4.
However, the differential amplifiers 12 have an in-phase input range consisting of the voltage range in which they can operate normally by application of voltage to their input terminals and are subject to restriction in regard to their input voltage range, depending on the device characteristics and power source voltage. For example, when a certain device is used as a differential amplifier 12, its in-phase input range, when the power source voltage VCC is +15V, is 14V. This in-phase input range differs somewhat depending on the device used to constitute differential amplifier 12 but in general is a value a little lower than the +15V of the power source voltage Vcc, so its value may be taken as 14V. On the other hand, since a large number of secondary batteries 11 are connected in series, as shown in FIG. 6, it may happen that the voltage Va of one terminal of a given secondary battery 11 is for example 200V with respect to ground; in such a case, too, it is necessary to ensure that the input voltage Vb does not exceed 14V, which is the in-phase input range. Consequently, in regard to R.sub.1 and R.sub.2 of FIG. 6, R.sub.2 /R.sub.1 must be made .ltoreq.14(200-14) and in this case the gain of the differential amplifier 12 has to be below R.sub.2 /R.sub.1 (=7/93). Accordingly, in order to match the input range of the above A/D converter, a gain trimmming amplifier 13 of gain R.sub.1/ 4R.sub.2 (=93/28) is required.
However, there was the problem that in the above construction costs were greatly increased by the need to provide gain trimmming amplifiers 13 for each differential amplifier 12 i.e. for each secondary battery 11.
Also, a background current flows from each secondary battery 11 to the differential amplifiers 12 provided for respective voltage measurement. If this background current was large, there was the problem that if the batteries were left for a long period without charging this resulted in over-discharge, causing deterioration of performance such as decrease of battery capacity and/or rise in internal resistance. There was also the problem that, if there were differences in the background current values between the series-connected secondary batteries 11, this resulted in large variability of capacity between the individual secondary batteries. If control was exercised based solely on the center value of capacity, some batteries would be found to be over-charged or over-discharged, making it necessary to use them in such a way that such over-charging/discharging did not occur and so giving rise to a reduction in the range of allowable use. Also, if left for a long period without charging, due to variability of the residual capacity, the reduced capacity of some secondary batteries resulted in even further discharging of the other secondary batteries, tending to produce deterioration of battery performance. Thus the challenges were presented of making the background current small and making the difference between the individual secondary batteries 11 small.
However, since in the above construction the differential amplifiers 12 were connected in common to ground GND, their equivalent circuit was as shown in FIG. 3(b). FIG. 3 shows by way of example a case in which nine secondary batteries 11 are connected in series. Taking the battery voltages of the individual secondary batteries as VB1 to VB9, the equivalent impedance at the differential amplifier 12 as R, the background currents from the individual secondary batteries 11 as I.sub.1 to I.sub.9, if for example VB1 to VB9 are assumed to be 15V and R to be 1 M.OMEGA., I.sub.1 =I.sub.9 =67.5 .mu.A, I.sub.2 =I.sub.8 =120.0 .mu.A, I.sub.3 =I.sub.7 =157.5 .mu.A, I.sub.4 =I.sub.6 =180.0 .mu.A, and I.sub.5 =187.5 .mu.A. Thus the background currents in the secondary batteries 11 are large with a maximum of 180.0 .mu.A and the difference in the background currents is a maximum of 120 .mu.A (this reaches 67% of the maximum background current value of 180.0 .mu.A). Thus there was the problem that the aforementioned challenges could not be met.
In the light of the above problems of the prior art, an object of the present invention is to provide a battery voltage measurement device wherein costs can be reduced by eliminating the need to provide gain trimmming amplifiers for the differential amplifiers used to measure the battery voltages and wherein the background currents can be made small and their variability also made small.